What is a Def Fluid?

What is Def Fluid?

Diesel engines have been around for more than 100 years, but it was only in the last decade when it has experienced tremendous growth and development. To minimize the harmful emissions by diesel engines to the air, various systems are installed to modern diesel engines to ensure that. Among them is the diesel exhaust fluid, which is also known as DEF fluid. What is DEF fluid? And what does it do for diesel engines?

This guide covers the basic information you need when using DEF fluid and how it helps diesel engines.

Key Takeaways

• DEF fluid is a formulation that is injected into the diesel engine exhaust system to produce a cleaner emission and reduce air pollution.
• DEF fluid is easily accessible and costs vary depending on the grade and brand.
• Def fluid is consumed at a rate of 50 to 1 for fuel consumption, but it varies depending on the tank size, vehicle mileage and efficiency rating.
• Store DEF fluid in a cool, dry place and away from direct sunlight.

DEF fluid and diesel engine

What is Def Fluid?

DEF fluid is a clear liquid that you inject into the exhaust system of diesel engines to minimize air pollution. In the next section, you will learn exactly how it does that, but this fluid consists of 67% deionized water. The rest of the formula consists of what is known as urea.

Urea is a natural byproduct resulting from the cell’s metabolic process. In nature, you can excrete this substance from the urine of mammals. However, the substance used in the DEF fluid formulation is synthetically derived in a laboratory.

DEF fluid is easily accessible in many gas stations, auto parts stores, home improvement stores, and in most large supermarkets. The cost of buying one differs depending on where you get it from and the container size.

What Does a DEF Fluid Do?

So, we’ve answered this question: what is DEF fluid? The next step is to determine its function and purpose.

Diesel exhaust fluid (DEF fluid) is crucial to how the diesel engine emission system works. When diesel engines were introduced into the market, there were several issues with the quality of its emissions. There were earlier attempts to improve the cleanliness of its emissions, but many of them failed. A common problem with older diesel engines is that they were prone to release soot and unburnt fuel.

There was the option to use a lean burn fuel-to-air ratio; however, this attempt was also unsuccessful because it resulted in the release of nitrogen oxide into the air. Nitrogen oxide was even more harmful to the environment!

Hence, urea was introduced as a solution to clean up the emissions of diesel engine exhaust systems. It is similar to nitrogen for its ability to bind to the exhaust without causing nitrogen release. This is how the Diesel Exhaust Fluid was developed with the help of de-ionized water to help transport urea. When DEF fluid combines with the hot exhaust gas stream, it causes the water to boil and release the urea. This process ignites a series of chemical reactions that results in the release of nothing but water and carbon dioxide.

Using DEF fluid is essentially the secret to a cleaner, more efficient exhaust emission for diesel engines, making it a must-have if you drive a diesel engine vehicle.

Diesel engines come with a gauge for the DEF tank. It will indicate when you’re almost out of DEF fluid, so you are aware when it is time to fill up again.

How Often Do You Need to Use DEF Fluid on a Diesel Engine?

The amount of DEF fluid you must use on a vehicle depends on its size, condition, and mileage. On average, a diesel engine vehicle consumes DEF fluid at a rate of 50 to 1 with diesel. It means that for every 50 gallons of diesel you consume, you will need 1 gallon of DEF fluid.

A well-maintained or new diesel engine vehicle will most likely require less DEF fluid per mile. It performs even better when you have an EcoDiesel Engine. If you’re wondering what is EcoDiesel engine, it is made with advanced diesel technology and efficiency.

How Long Does DEF Fluid Last?

There is no one answer because it varies according to several factors. Industry experts have claimed that the DEF fluid mileage is directly proportional to the fuel consumption. Therefore, the higher your fuel economy rating, the longer the DEF fluid lasts.

There is no direct numbers that will indicate exactly how long your DEF fluid last, but using the information above, you can understand the importance of improving your fuel economy as it correlates to the DEF fluid performance.

What Happens If You Run Out Of Diesel Exhaust Fluid?

Since DEF fuel is consumable like diesel fuel, many vehicle owners are curious what would happen to their vehicle if they run out of this fluid. It’s an important question since many car owners don’t stock up on them given how long it takes to consume a full tank. Therefore, it is possible for a diesel engine vehicle to run out of DEF fluid.

So, what happens if this occurs? Modern diesel engines systems are designed to activate the limp mode on your engine via the powertrain control module, which is basically forcing your vehicle to reduce its top speed without DEF fluid. In addition, there will be clogging in the exhaust system, risking more damage.

Therefore, driving without DEF fluid must be avoided at all times. Modern diesel engine vehicles have a gauge indicator that will warn you ahead of time when you’re low in DEF fluid. It will give you time to fill up the tank to prevent the possibility of driving on an empty DEF fluid tank.

DEF Fluid Tank

Do’s and Don’ts When Using Def Fluid

Take note of these guidelines when using DEF fluid for best results.


• Fill the DEF tank correctly. You can see the markings on the nozzle and outlet.
• Always clean your DEF equipment and make sure there are no dust or dirt in the nozzle. DEF fluid is highly sensitive to air-borne contaminants and residue. Contaminated DEF fluid affects the SCR system performance and cause engine damage over time.
• Store DEF fluid properly in dedicated containers. Keep it in a cool, dry place and away from direct sunlight.
• Use a digital refractometer to monitor the DEF quality. The best DEF fluid quality is 32.5-37% of urea.
• You can store DEF fluid for up to one year in ideal conditions and using the proper storage recommendations.
• The freezing temperature for DEF fluid is 12 degrees. If you are freezing DEF fluid, it is fine as it won’t affect the performance. However, store it in expandable containers.


• Avoid pouring the DEF fluid into the diesel tank (or vice versa). Make sure each container is clearly marked so you don’t confuse these two fluids.
• Do not store DEF fluid in unsealed containers because of the chance of evaporating.
• Do not get your DEF fluid on clothes and skin. If you do get it on your skin by accident, run it over cool water and use soap. Rinse thoroughly.
• Do not use DEF fluid that has been mixed with water or other substances.

Final Thoughts

Now that you know what is DEF fluid, you can use that information to make the right choice for your diesel engines. Consider the quality and brand when investing in DEF fluid. While they are technically the same, you must be cautious where you get your DEF fluid to ensure it has no contaminants and the best quality for your vehicle’s exhaust system.


Does Def Fluid help fuel economy?

Engines that employ the use of DEF fluid has shown dramatic improvements in fuel economy, with an estimated increase of 3-5% in fuel efficiency. When using DEF, the SCR system operates with a higher efficiency by promoting better exhaust gas recirculation and boosting overall efficiency.

What happens if you put DEF fluid in your fuel tank?

It will cause a major system failure because the DEF fluid will clog the injectors and damage the seals in the fuel system. Therefore, be very careful when filling the DEF fluid into the correct tank.

Does DEF really help the environment?

DEF fluid helps reduce harmful nitrogen oxide emissions into the environment. In addition, it contributes to reducing particulate matter emissions, which eliminates air quality.

Is DEF fluid harmful to humans?

DEF fluid is non-toxic and non-flammable. It isn’t harmful to humans, but avoid contact with your skin and clothes. Follow the recommended handling and storage for DEF fluid to avoid contact with the skin.

Why is DEF fluid so expensive?

The cost of DEF fluid varies depending on where you buy it. However, higher quality DEF fluid has a higher urea concentration, which also means a higher grade DEF fluid.


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